The present invention relates to a positioning device which utilizes a radio wave from a transmission source, a method of controlling a positioning device, and a recording medium having a program for controlling a positioning device recorded thereon.
A positioning system has been used in practice which locates the present position of a GPS receiver utilizing a satellite navigation system such as a global positioning system (GPS).
The GPS receiver receives a clear and acquisition or coarse and access (C/A) code which is one type of pseudo-random noise code (hereinafter called “PN code”) carried on a radio wave from a GPS satellite (hereinafter called “satellite radio wave”) based on a navigation message indicating the orbit of the GPS satellite and the like (including almanac (approximate satellite orbital information), ephemeris (precise satellite orbital information), and the like). The C/A code is a code which forms the basis for positioning.
The GPS receiver specifies the GPS satellite which has transmitted the C/A code, and calculates the distance (pseudo-range) between the GPS satellite and the GPS receiver based on the phase of the C/A code (code phase), for example. The GPS receiver locates the position of the GPS receiver based on the pseudo-range between the GPS receiver and each of three or more GPS satellites and the position of each GPS satellite in the satellite orbit. For example, the C/A code has a bit rate of 1.023 Mbps and a code length of 1023 chips. Therefore, it is considered that the C/A codes line up in units of about 300 kilometers (km) over which a radio wave advances in 1 millisecond (ms). Therefore, the pseudo-range can be calculated by calculating the number of C/A codes between the GPS satellite and the GPS receiver from the position of the GPS satellite in the satellite orbit and the approximate position of the GPS receiver. In more detail, the pseudo-range can be calculated by calculating one cycle (1023 chips) of the C/A code (integer portion of the C/A code), and specifying the phase of the C/A code (fraction portion of the C/A code). The integer portion of the C/A code can be estimated when the approximate position of the GPS receiver has a specific accuracy (e.g. 150 km or less). Therefore, the GPS receiver can calculate the pseudo-range by specifying the phase of the C/A code.
The GPS receiver correlates the received C/A code with a C/A code replica generated in the GPS receiver, accumulates the correlation values, and specifies the phase of the C/A code when the correlation cumulative value has reached a specific level. The GPS receiver performs the correlation process while changing the phase of the C/A code replica and the frequency.
However, when the satellite radio wave received from the GPS satellite is an indirect wave which reaches the GPS receiver after being reflected by a building or the like (hereinafter called “indirect wave”), the GPS receiver cannot accurately specify the phase of the C/A code.
In order to deal with this problem, technology has been proposed in which a GPS receiver with a communication function stores map data with multipath frequency region information, and determines whether or not the present position acquired by positioning is included in the multipath frequency region. When a base station with which the GPS receiver communicates is located in an urban area, the GPS receiver determines whether or not the base station is located in the multipath frequency region (e.g. JP-A-2001-272450).
However, the above technology has a first problem in which it is necessary for the GPS receiver to store the map data or communicate with the base station. As a second problem, since a poor reception state also occurs in an environment other than the multipath environment and the multipath environment varies, the position may not be determined while effectively using satellite signals corresponding to various reception environments when merely determining the multipath environment.